1. Field of the Invention
The present invention relates to an inner rotor type molded motor.
2. Description of the Related Art
A known molded motor has a motor frame made of insulating molding resin covering a stator, and a rotor is disposed on a center-axis side of the stator in a rotatable manner. The stator includes a stator core and a coil formed around a portion of the stator core which serves as a main magnetic path. The rotor includes a shaft centered on the center axis of the molded motor. The shaft is supported by ball bearings disposed at or near axial ends of the stator. Since the resin molding the stator forms the motor frame, the molded motors do not require a component that functions or defines an outer casing of the motor. Thus, the number of the components of the molded motor and the manufacturing cost thereof can be reduced.
Typical molded motors use brushless DC motors. The brushless DC motors are usually controlled by pulse width modulation (hereinafter, simply referred to as PWM) control using an inverter circuit.
PWM control may cause electrical erosion in molded motors using brushless DC motors. This is because potentials at brackets which are usually made of metal and support the bearings, respectively, are “floating” with respect to the molding resin forming the motor frame and are therefore unstable.
In a case where a power-supply voltage is switched at high frequencies as in PWM control, for example, a potential at a load-side bracket and a potential at an anti-load-side bracket may become different due to a change in a distance between the stator and the rotor or due to a voltage change at a neutral point of a bridge circuit caused by PWM control. Thus, a potential difference is generated between an outer ring of the load-side ball bearing of the same potential as the load-side bracket and an outer ring of the anti-load-side ball bearing of the same potential as the anti-load-side bracket, resulting in a circulation current flowing through inner rings of the respective ball bearings and the shaft of the rotor. This circulation current causes discharge between the inner ring and the outer ring in each of ball bearing. The discharge makes surfaces of the inner ring, the outer ring, and a ball uneven or rough in each ball bearing. The ball bearing having an uneven or rough surface generates an abnormal sound, and the life thereof is shortened.